EGU2020-13617, updated on 12 Jun 2020
EGU General Assembly 2020
© Author(s) 2020. This work is distributed under
the Creative Commons Attribution 4.0 License.

Flocculation of microplastic and cohesive sediment in natural seawater

Thorbjørn Joest Andersen, Stiffani Rominikan, Ida Stuhr Laursen, Kristoffer Hofer Skinnebach, Nynne Zaza Grube, Soeren Roger Jedal, Simon Nyboe Laursen, and Mikkel Fruergaard
Thorbjørn Joest Andersen et al.
  • University of Copenhagen, Geosciences and Natural Resource Management, Copenhagen, Denmark (

The flocculation of combinations of microplastic particles (MP) and natural cohesive sediment has been investigated in a laboratory setup using unfiltered seawater. The experiments were conducted in order to test the hypothesis that MP may flocculate in estuarine and marine environments with natural organic and inorganic particles. MP particles in the size-range 63 – 125 µm were incubated with suspensions of local untreated seawater and untreated fine-grained sediment (< 20µm) collected from a tidal mudflat. Settling experiments were carried out with both a floc-camera video equipment (PCam) and conventional settling tubes.

Flocculation and sedimentation of MP-particles of PVC have been investigated as well as particles from high density polypropylene which is used in certain fishing gear. The studies have generally confirmed our hypothesis that microplastics are incorporated into aggregates along with other natural particles, thus settling faster than they would as single particles. The exact aggregation mechanisms still remains to be revealed but the general cohesiveness of fine-grained natural particles, organic particles as well as particulate and dissolved organic polymers are believed to be responsible for the flocculation. A strong effect of salt ions was also observed, confirming the classical concept of increased flocculation of fine-grained particles as they are transported from fresh-water to estuarine and marine waters.

The implication of the aggregation is that primary MP from land-based sources are likely to flocculate with other suspended particles, especially as they enter saline waters. The particles are therefore expected to deposit close to the sources, typically rivers. This applies to both micro-plastic particles that are denser than seawater but also to low-density plastic types which should otherwise float. However, secondary MP may be formed by disintegration of plastic anywhere and these MP particles could therefore settle wherever there is plastic present at the sea surface, for example under the ocean gyres where plastic is known to accumulate. Here, too, interaction with other particles in the water column is expected, but the concentration of natural particles is much lower than in coastal waters and it may be that the transport of natural organic and inorganic particles will start to be modified if the concentration of plastic in the marine environment continues to rise.

How to cite: Andersen, T. J., Rominikan, S., Laursen, I. S., Skinnebach, K. H., Grube, N. Z., Jedal, S. R., Laursen, S. N., and Fruergaard, M.: Flocculation of microplastic and cohesive sediment in natural seawater, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-13617,, 2020


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displays version 1 – uploaded on 02 May 2020
  • CC1: More explanation about slides, Gholamreza Shiravani, 06 May 2020

    Hi dear author,

    could you tell me:

    -in slide 4, the y-axis is percentage of what?and x-axis is the settling velocity of sediment or MP or MP-sediment mixture? What does mean "Control" test precisely (I mean how much is the MP-concentration and fine sediments in this sample)?Are the MP-concentration in whol medium (sediment+water) or only in water?

    -in slide 5, from which depth were the samples sampled?How much is the MP-concentration in each curve? are always the MP-concentration 1 mg/l?

    -Did you publish a paper or a report about your experiments?If yes, I will be happy to send its link by this message.

    Thans so much and if you tell me how do you define the MP-concentration as Mass(%), I will understand most of your presentation.


    • AC1: Reply to CC1, Thorbjørn Joest Andersen, 06 May 2020

      Hi Gholamreza, thanks for your interest in our work.

      So, slide 4, the y-axis is simply the mass % so the figure shows the distribution of settling velocities of the total suspensions. The control is a suspension of 100 mg/l cohesive sediment with no plastic added.

      Slide 5: sub-samples are taken from the bottom of the settling tube. So, after 64 min all the water has been sub-sampled. Samples taken at the beginning of the experiments contain fast and slow settling flocs and samples from the end of the experiment only flocs settling very slowly.

      We are able to discern between the natural sediment and the microplastic because we make sure they are different sizes. The natural sediment was below 20 micron whereas the MP was 63-125 micron. That enables us to discern the two types of particles in the grain size spectra (slide 5). And the MP fraction of each subsample is plotted in slide 6.

      We have submitted a manuscript including the data shown here but has not been published yet. Feel free to send me an email and I will make sure you get a copy once its published. My email is 

      Cheers, Thorbjørn 

      • CC2: Reply to AC1, Gholamreza Shiravani, 06 May 2020

        Thanks for your answers and I 'll be happy if you send me more material about your research.

        I wrote you an email. My email is:


        Cheers, Reza